The invention relates generally to electrical component test handlers and more particularly to a low noise multi-lead contact made by modifying lead frames of the type which are mounted on holders used in electronic component test handling systems. The invention further relates to a method of modifying prior art lead frames to reduce the inductance of selected leads.
Electrical component test handling systems are designed to subject packaged integrated circuits and similar electronic components to various electrical and environmental tests. Connections are made to selected interconnect pads or pins on the components by means of resilient leads which are bundled together on lead frames containing multiple elongated, uniform, parallel conductors, or leads. Lead frames are generally mounted on holders which orient the lead frames to make convenient contact with the test components, and through which external connections are made to the conductors on the lead frames. During tests the conductors on the lead frames are urged into temporary contact with the interconnect pins on the component package.
Individual thin-strip conductors on a lead frame can exhibit high inductance if high-frequency test signals are used to test components. Such high-frequency test signals, for example, in the 50-100 MHz range, may cause relatively high current gradients in the power supply signal carried to a test component through a lead frame's power or ground conductors. Because of the inductance of the lead frame conductors which supply power to test components, current gradients in the power and ground conductors during high frequency testing produces undesirable voltage fluctuations in the power supply. The consequence is noise and a degradation of the test results.
The inductance of a thin-strip conductor can be reduced by enlarging the cross section of the conductor. In theory, the inductance of the power or ground conductor on a lead frame could be reduced by simply enlarging that conductor. However, lead frames used in component test handlers cannot readily be modified to enlarge a selected conductor without compromising the lead frame's compatibility with related test equipment. Lead frames are mounted on lead frame holders when in use. The lead frame holders are expensive, compared to the cost of lead frames, and the lead frames are made to be replaceable and interchangeable. Most conventional lead frame holders are only compatible with lead frames made up of conductors that are uniform in size, and which have uniform separation between the conductors. The holders can be reconfigured to accept lead frames of different sizes, with different shapes and sizes of uniform conductors. But a lead frame with one or two enlarged conductors, for example, to reduce the inductance of a selected conductor, would not fit a conventional lead frame holder. Consequently, it is not possible to solve the problem presented by excessive inductance in a selected conductor on a lead frame by simply increasing the size or cross section of that selected conductor since the lead frame would then be incompatible with a conventional lead frame holder.
The owners of electronic component test equipment generally have a substantial investment in lead frame holders. The holders are used repeatedly to test production runs of electronic components. If a particular component requires high frequency testing in which the inherent inductance of one or two selected leads on a lead frame will degrade test results, it is not always economically feasible to design, manufacture, and use a custom lead frame having power or ground conductors that are larger than the other leads on the frame. Such an approach is prohibitively costly because it would require both a custom lead frame and a custom holder. Since electronic components are made in the widest possible variety of configurations and pin locations, with ground-and power-supply pins at differing locations on the packages, it is better if lead frames and holders can be reconfigured easily to test a variety of components. It is simply not cost effective to manufacture a large number of custom lead frames and holders to perform tests on quantities of a single component if the lead frames, and particularly the holders, cannot be re-used. Accordingly, there is a need in the industry for a solution to the high frequency inductance problem using lead frames which are compatible with conventional lead frame holders.
It would, therefore, be advantageous to provide an improved low noise multi-lead contact compatible with the present generation of lead frame holders used in component test handlers, wherein the improved multi-lead contact solves the problem of power supply voltage fluctuations caused by the inductance of selected leads supplying power to test component.
It would also be advantageous to provide a modified lead frame having multiple parallel conductors in which selected conductors have reduced inductance and other conductors are shielded from high frequency noise.
It would also be advantageous to provide a methodology for modifying prior art lead frames used in component test handlers to reduce the effective inductance of selected leads on the lead frame while leaving the resultant lead frame usable with existing lead frame holders.
Accordingly, a low noise multi-lead contact is provided, adapted from a lead frame, for mounting on a lead frame holder of the type used in electrical component test handlers. Such handlers are designed to selectively complete multiple simultaneous electrical connections with interconnect points on components to test the components. The lead frame from which the low noise multi-lead contact is adapted includes multiple parallel conductors for carrying signals from test equipment to a component. Each such lead frame has opposed inner and outer sides and is mounted on a holder in a predetermined orientation. The inner side of the lead frame is the side which faces the major mass of the holder when mounted thereon and the outer side is opposite the inner side. The low noise multi-lead contact adapted from the lead frame comprises, on the outer side of the lead frame, a first shield formed of an expanse of conductive material overlying a portion of the lead frame. The first shield is electrically coupled to one or more selected conductors on the lead frame, such Selected conductors being referred to as the shield-coupled conductors. The shield-coupled conductors have reduced inductance during selected component tests. The other conductors on the lead frame, referred to herein as shield-isolated conductors, are electrically isolated from the first shield overlying the outer side of the lead frame. A second shield formed of an expanse of conductive material is provided on the inner side of the lead frame. The second shield overlies a portion of the lead frame and is not coupled to any of the conductors. That is, the second shield is electrically isolated from all conductors on the lead frame.
In its preferred form the first and second shields, attached respectively to the outer and inner sides of the lead frame, are each affixed to the lead frame by a narrow band of attachment which extends laterally across the lead frame generally perpendicular to the parallel conductors. The shields are unattached to the lead frame except along the narrow band of attachment. In that way, the lead frame, when mounted on a holder, is flexible and resilient and can be bent when force is exerted perpendicularly against the lead frame to urge the conductors into contact with interconnect points on a component.
More particularly, the first shield, on the outer side of the lead frame, is large enough to overlie both the shield-coupled conductors and the shield-isolated conductors on the lead frame. In order to electrically insulate the shield-isolated conductors from the first shield, a first insulating layer is provided, extending between the first shield and the shield-isolated conductors. A second insulating layer, which extends over the inner side of the lead frame, extends between the conductors and the second shield, electrically insulating the second shield from all the conductors on the lead frame. The first and second insulating layers are preferably formed of an insulating sheet material having a thickness generally in the range of 1-mil to 8-mils. The insulating layers are preferably attached to the lead frame by adhesive.
An elongated electrical connection is established between the shield-coupled conductors on the lead frame and the first shield, the connection extending over a major portion of the length of the shield-coupled conductors. The elongated electrical connection is in the form of a conductive spacer, which is a thin sheet of conductive material or foil, preferably having a thickness which is generally equal to the thickness of the first insulating layer. The spacer is positioned between the shield coupled conductors and the first shield. The spacer provides an electrical path between the one or more shield-coupled conductors and the first shield on the outer side of the lead frame.
Electrical connections are made to individual conductors on a lead frame via contact points extending through the lead frame holder, the contact points making physical contact with selected conductors on the inner side of the lead frame. In order to allow such connections to be made to one or more of the shield-coupled conductors on the lead frame, a connection path is provided through the second shield and second insulating layer on the inner side of the lead frame. When the lead frame is mounted on a holder, suitable contact points are selectively extended from the holder through the connection path to contact selected shield-coupled conductors on the lead frame.
The invention also includes a method of modifying a lead frame of the type which is mounted on a holder used in component test handlers, such lead frames being designed to selectively complete multiple simultaneous electrical connections with interconnect points on test components. Such lead frames have multiple parallel conductors partially covered by adherent insulating material which extends laterally between adjacent conductors to secure the conductors to one another. The lead frames are mounted on the holders in a predetermined orientation and have inner and outer sides determined by the mounting orientation. The outer side faces away from the major mass of the holder, and the inner side faces toward the major mass of the holder, when the lead frame is mounted on a holder. The method of modifying the lead frame comprises the following steps: a) on the outer side of the lead frame, remove an area of adherent insulation to expose the outer side of one or more selected conductors; b) on the outer side of the lead frame, outside the area where the adherent insulation is removed in step a), position insulating sheet material over a major portion of the lead frame to provide a first insulating layer overlying the lead frame; c) on the outer side of the lead frame, position a first shield, formed of an expanse of conductive material, over the first insulating layer provided in step b), including over the portions of the selected conductors exposed in step a), and establish an electrical connection between the first shield and such selected conductors, whereby the first shield is electrically coupled to the selected conductors exposed in step a); d) on the inner side of the lead frame, position insulating sheet material over a major portion of the lead frame to provide a second insulating layer overlying the lead frame, the second insulating layer including a opening therethrough adjacent the selected conductors, whereby selected electrical connections can be established through the opening between a holder and the selected conductors; and e) on the inner side of the lead frame, position a second shield, formed of an expanse of conductive material, over the second insulating layer, except over the opening, such that the second shield is electrically isolated from the lead frame by the second insulating layer.
In its preferred form, the method of modifying a lead frame in accordance with the present invention includes the additional step, on the outer side of the lead frame, of securing the first shield to the first insulating layer by means of a narrow band of attachment extending laterally across the lead frame, generally perpendicular to the parallel conductors. Also, on the inner side of the lead frame, the second shield is secured to the second insulating layer by means of a narrow band of attachment extending laterally across the lead frame, generally perpendicular to the parallel conductors. As such, the two aforementioned securing steps leave the first and second shields unattached to the respective first and second insulating layers except along such narrow bands of attachment. That allows the lead frame to remain flexible, despite the attachment of the conductive shields.
An additional step in the preferred method is to install a conductive element between the first outer shield and the selected conductors to ensure that an electrical connection is established between the first shield and the selected conductors. The element installed is preferably a conductive spacer having generally the same thickness as the first insulating layer. It is installed on the outer side of the lead frame overlying the portions of the selected conductors exposed in step a). The spacer extends between and electrically connects the selected conductors and the first shield. The selected conductors on the lead frame include at least one of the conductors which supplies power to the test component. In other words, the selected conductors include either the power or the ground lead carrying power to the component. The power or ground lead is thus electrically coupled to the first (outer) shield, thereby greatly increasing the cross section of the power or ground lead, reducing its inductance.